A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of a die, one die, or several dies) on a substrate (e.g. a silicon wafer) Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
A theoretical estimate of the limits of pattern printing can be given by the Rayleigh criterion for resolution as shown in equation (1):
                    CD        =                              k            1                    *                      λ            NA                                              (        1        )            
where λ is the wavelength of the radiation used, NA is the numerical aperture of the projection system used to print the pattern, k1 is a process dependent adjustment factor, also called the Rayleigh constant, and CD is the feature size (or critical dimension) of the printed feature. It follows from equation (1) that reduction of the minimum printable size of features can be obtained in three ways: by shortening the exposure wavelength λ, by increasing the numerical aperture NA or by decreasing the value of k1.
In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
EUV radiation may be produced using a plasma. A radiation system for producing EUV radiation may include a laser for exciting a fuel to provide the plasma, and a source collector module for containing the plasma. The plasma may be created, for example, by directing a laser beam at a fuel, such as particles of a suitable material (e.g. tin), or a stream of a suitable gas or vapor, such as Xe gas or Li vapor. The resulting plasma emits output radiation, e.g., EUV radiation, which is collected using a radiation collector. The radiation collector may be a mirrored normal incidence radiation collector, which receives the radiation and focuses the radiation into a beam. The source collector module may include an enclosing structure or chamber arranged to provide a vacuum environment to support the plasma. Such a radiation system is typically termed a laser produced plasma (LPP) source.
A lithographic apparatus generally includes an illumination system. The illumination system receives radiation from a source, for example an excimer laser or an extreme ultra violet radiation source, and provides a radiation beam (sometimes referred to as a “projection” beam) which is incident upon a patterning device. The radiation beam is patterned by the patterning device, and is then projected by a projection system onto a substrate.
It is known in the art of lithography that an image of the patterning device projected onto a substrate can be improved by providing the radiation beam with an appropriate illumination mode. An illumination mode is a spatial intensity distribution of the radiation beam in a pupil plane of the illumination system, centred with respect to an axis or optical axis of the illumination system. At the plane of the patterning device (a field plane of the illumination system) such a spatial intensity distribution corresponds to a distribution of angles of incidence of incident radiation, referred to as an angular intensity distribution of radiation. A desired illumination mode may, for example, be a conventional illumination mode having a central irradiated portion in the pupil plane or an off axis illumination mode having one or more isolated, off axis irradiated portions in the pupil plane. Accordingly, an illumination system of a lithographic apparatus typically includes an intensity distribution adjustment apparatus arranged to direct, shape and control the radiation beam in the illumination system such that a selected illumination mode can be achieved.
The prior art describes various intensity distribution adjustment apparatus arranged to control the illumination beam so as to achieve a desired illumination mode. For example, a zoom-axicon device (a combination of a zoom lens and an axicon) can be used to create an annular illumination mode, whereby the inner and outer radii of the annular intensity distribution in the pupil plane are controllable. The magnitude of the inner and outer radii is usually indicated by σinner and σouter respectively. These numbers represent respectively the ratio of the inner radius and the ration of the outer radius to the radius corresponding to the numerical aperture of the projection system. A zoom-axicon device generally comprises multiple refractive optical components that are independently movable. A zoom-axicon device is therefore not suitable for use with EUV radiation (e.g. radiation at 13.5 nm or with a wavelength between 5 and 20 nm), because radiation at these wavelengths is strongly absorbed as it passes through refractive materials.
Spatial filters may be used to create illumination modes. For example, a spatial filter with two opposite, off axis openings corresponding to a dipole illumination mode may be provided in a pupil plane of the illumination system in order to generate a dipole illumination mode. The spatial filter may be removed and replaced by a different spatial filter when a different illumination mode is required. However, spatial filters block a considerable proportion of the radiation beam, thereby reducing the intensity of the radiation beam when it is incident upon the patterning device. Known EUV sources struggle to provide EUV radiation at an intensity which is sufficient to allow a lithographic apparatus to operate efficiently. Therefore, it is not desirable to block a considerable portion of the radiation beam when forming the illumination mode.